Fix cast in `solve_as_val`.

Implicit typeclass instances are going to be really annoying if we
actually end up adopting these casts.

theories/heap_lang/tactics.v

@@ -226,7 +226,10 @@ Ltac solve_as_val :=
   match goal with
   | |- AsVal ?e =>
      let e' := W.of_expr e in change (is_Some (to_val (W.to_expr e')));
-     apply W.to_val_is_Some, (bool_decide_unpack _); exact (I <: bool_decide (is_Some (W.to_val e')))
+     apply W.to_val_is_Some, (bool_decide_unpack _);
+     match goal with [|-Is_true (@bool_decide ?P ?dec)] =>
+       exact (I <: Is_true (@bool_decide P dec))
+     end
   end.
 Hint Extern 10 (AsVal _) => solve_as_val : typeclass_instances.